class Proc
Practical examples and pitfalls for Proc
Proc examples: choose the callback contract deliberately
Practical notes by Ruby-Doc.org
Decide how many arguments the callback accepts
A lambda is a useful default for a callback with a firm argument list. Passing the wrong number of arguments raises an error at the call site. An ordinary proc is more permissive with positional arguments; a missing argument can become nil, and an extra argument can be ignored.
strict = ->(left, right) { [left, right] }
loose = proc { |left, right| [left, right] }
p loose.call("parcel")
begin
strict.call("parcel")
rescue ArgumentError
puts "two arguments required"
end
["parcel", nil] two arguments required
Neither behavior is automatically the right choice for every block. A callback used by your own service object may benefit from strict arguments, while a block passed to an iterator can intentionally ignore values it does not need. Document the contract where the callback is accepted, especially when several different implementations can be supplied.
Notice which local state the closure retains
A proc can keep access to a surrounding local variable after the method that created it has returned. Here each call to the factory creates a separate running total. Calls to the first accumulator share its total, while the second accumulator starts from its own initial value.
def accumulator(initial)
total = initial
->(amount) { total += amount }
end
first = accumulator(10)
second = accumulator(100)
p [first.call(2), first.call(3), second.call(2)]
[12, 15, 102]
That retained state can be useful, but it also means the callback is doing more than computing a result from its arguments. Reusing the same callback in two places will reuse its captured state. If that is surprising in the application, use a stateless callback or a named object with an explicit lifecycle.
Keep control flow easy to follow
return has different behavior in lambdas and ordinary procs. Avoid moving a block containing return into a stored callback without checking what it will return from. A lambda returns from its own invocation; an ordinary proc can attempt to return from the method where it was created.
Small callbacks are easiest to inspect. Once a callback owns several mutable variables or needs cleanup around external resources, a class with a call method can make those responsibilities clearer. The caller can keep the same calling convention while the implementation gains a place for names and focused tests.
API reference: Proc API reference
Related: Method · Enumerator
A Proc object is an encapsulation of a block of code, which can be stored in a local variable, passed to a method or another Proc, and can be called. Proc is an essential concept in Ruby and a core of its functional programming features.
square = Proc.new {|x| x**2 } square.call(3) #=> 9 # shorthands: square.(3) #=> 9 square[3] #=> 9
Proc objects are closures, meaning they remember and can use the entire context in which they were created.
def gen_times(factor) Proc.new {|n| n*factor } # remembers the value of factor at the moment of creation end times3 = gen_times(3) times5 = gen_times(5) times3.call(12) #=> 36 times5.call(5) #=> 25 times3.call(times5.call(4)) #=> 60
Creation
There are several methods to create a Proc
-
Use the
Procclass constructor:proc1 = Proc.new {|x| x**2 }
-
Use the
Kernel#procmethod as a shorthand ofProc.new:proc2 = proc {|x| x**2 }
-
Receiving a block of code into proc argument (note the
&):def make_proc(&block) block end proc3 = make_proc {|x| x**2 }
-
Construct a proc with lambda semantics using the
Kernel#lambdamethod (see below for explanations about lambdas):lambda1 = lambda {|x| x**2 }
-
Use the Lambda proc literal syntax (also constructs a proc with lambda semantics):
lambda2 = ->(x) { x**2 }
Lambda and non-lambda semantics
Procs are coming in two flavors: lambda and non-lambda (regular procs). Differences are:
-
In lambdas,
returnandbreakmeans exit from this lambda; -
In non-lambda procs,
returnmeans exit from embracing method (and will throwLocalJumpErrorif invoked outside the method); -
In non-lambda procs,
breakmeans exit from the method which the block given for. (and will throwLocalJumpErrorif invoked after the method returns); -
In lambdas, arguments are treated in the same way as in methods: strict, with
ArgumentErrorfor mismatching argument number, and no additional argument processing; -
Regular procs accept arguments more generously: missing arguments are filled with
nil, singleArrayarguments are deconstructed if the proc has multiple arguments, and there is no error raised on extra arguments.
Examples:
# +return+ in non-lambda proc, +b+, exits +m2+. # (The block +{ return }+ is given for +m1+ and embraced by +m2+.) $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { return }; $a << :m2 end; m2; p $a #=> [] # +break+ in non-lambda proc, +b+, exits +m1+. # (The block +{ break }+ is given for +m1+ and embraced by +m2+.) $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { break }; $a << :m2 end; m2; p $a #=> [:m2] # +next+ in non-lambda proc, +b+, exits the block. # (The block +{ next }+ is given for +m1+ and embraced by +m2+.) $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1 { next }; $a << :m2 end; m2; p $a #=> [:m1, :m2] # Using +proc+ method changes the behavior as follows because # The block is given for +proc+ method and embraced by +m2+. $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { return }); $a << :m2 end; m2; p $a #=> [] $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { break }); $a << :m2 end; m2; p $a # break from proc-closure (LocalJumpError) $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&proc { next }); $a << :m2 end; m2; p $a #=> [:m1, :m2] # +return+, +break+ and +next+ in the stubby lambda exits the block. # (+lambda+ method behaves same.) # (The block is given for stubby lambda syntax and embraced by +m2+.) $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { return }); $a << :m2 end; m2; p $a #=> [:m1, :m2] $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { break }); $a << :m2 end; m2; p $a #=> [:m1, :m2] $a = []; def m1(&b) b.call; $a << :m1 end; def m2() m1(&-> { next }); $a << :m2 end; m2; p $a #=> [:m1, :m2] p = proc {|x, y| "x=#{x}, y=#{y}" } p.call(1, 2) #=> "x=1, y=2" p.call([1, 2]) #=> "x=1, y=2", array deconstructed p.call(1, 2, 8) #=> "x=1, y=2", extra argument discarded p.call(1) #=> "x=1, y=", nil substituted instead of error l = lambda {|x, y| "x=#{x}, y=#{y}" } l.call(1, 2) #=> "x=1, y=2" l.call([1, 2]) # ArgumentError: wrong number of arguments (given 1, expected 2) l.call(1, 2, 8) # ArgumentError: wrong number of arguments (given 3, expected 2) l.call(1) # ArgumentError: wrong number of arguments (given 1, expected 2) def test_return -> { return 3 }.call # just returns from lambda into method body proc { return 4 }.call # returns from method return 5 end test_return # => 4, return from proc
Lambdas are useful as self-sufficient functions, in particular useful as arguments to higher-order functions, behaving exactly like Ruby methods.
Procs are useful for implementing iterators:
def test [[1, 2], [3, 4], [5, 6]].map {|a, b| return a if a + b > 10 } # ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ end
Inside map, the block of code is treated as a regular (non-lambda) proc, which means that the internal arrays will be deconstructed to pairs of arguments, and return will exit from the method test. That would not be possible with a stricter lambda.
You can tell a lambda from a regular proc by using the lambda? instance method.
Lambda semantics is typically preserved during the proc lifetime, including &-deconstruction to a block of code:
p = proc {|x, y| x } l = lambda {|x, y| x } [[1, 2], [3, 4]].map(&p) #=> [1, 3] [[1, 2], [3, 4]].map(&l) # ArgumentError: wrong number of arguments (given 1, expected 2)
The only exception is dynamic method definition: even if defined by passing a non-lambda proc, methods still have normal semantics of argument checking.
class C define_method(:e, &proc {}) end C.new.e(1,2) #=> ArgumentError C.new.method(:e).to_proc.lambda? #=> true
This exception ensures that methods never have unusual argument passing conventions, and makes it easy to have wrappers defining methods that behave as usual.
class C def self.def2(name, &body) define_method(name, &body) end def2(:f) {} end C.new.f(1,2) #=> ArgumentError
The wrapper def2 receives body as a non-lambda proc, yet defines a method which has normal semantics.
Conversion of other objects to procs
Any object that implements the to_proc method can be converted into a proc by the & operator, and therefore can be consumed by iterators.
class Greeter def initialize(greeting) @greeting = greeting end def to_proc proc {|name| "#{@greeting}, #{name}!" } end end hi = Greeter.new("Hi") hey = Greeter.new("Hey") ["Bob", "Jane"].map(&hi) #=> ["Hi, Bob!", "Hi, Jane!"] ["Bob", "Jane"].map(&hey) #=> ["Hey, Bob!", "Hey, Jane!"]
Of the Ruby core classes, this method is implemented by Symbol, Method, and Hash.
:to_s.to_proc.call(1) #=> "1" [1, 2].map(&:to_s) #=> ["1", "2"] method(:puts).to_proc.call(1) # prints 1 [1, 2].each(&method(:puts)) # prints 1, 2 {test: 1}.to_proc.call(:test) #=> 1 %i[test many keys].map(&{test: 1}) #=> [1, nil, nil]
Orphaned Proc
return and break in a block exit a method. If a Proc object is generated from the block and the Proc object survives until the method is returned, return and break cannot work. In such case, return and break raises LocalJumpError. A Proc object in such situation is called as orphaned Proc object.
Note that the method to exit is different for return and break. There is a situation that orphaned for break but not orphaned for return.
def m1(&b) b.call end; def m2(); m1 { return } end; m2 # ok def m1(&b) b.call end; def m2(); m1 { break } end; m2 # ok def m1(&b) b end; def m2(); m1 { return }.call end; m2 # ok def m1(&b) b end; def m2(); m1 { break }.call end; m2 # LocalJumpError def m1(&b) b end; def m2(); m1 { return } end; m2.call # LocalJumpError def m1(&b) b end; def m2(); m1 { break } end; m2.call # LocalJumpError
Since return and break exits the block itself in lambdas, lambdas cannot be orphaned.
Anonymous block parameters
To simplify writing short blocks, Ruby provides two different types of anonymous parameters: it (single parameter) and numbered ones: _1, _2 and so on.
# Explicit parameter: %w[test me please].each { |str| puts str.upcase } # prints TEST, ME, PLEASE (1..5).map { |i| i**2 } # => [1, 4, 9, 16, 25] # it: %w[test me please].each { puts it.upcase } # prints TEST, ME, PLEASE (1..5).map { it**2 } # => [1, 4, 9, 16, 25] # Numbered parameter: %w[test me please].each { puts _1.upcase } # prints TEST, ME, PLEASE (1..5).map { _1**2 } # => [1, 4, 9, 16, 25]
it
it is a name that is available inside a block when no explicit parameters defined, as shown above.
%w[test me please].each { puts it.upcase } # prints TEST, ME, PLEASE (1..5).map { it**2 } # => [1, 4, 9, 16, 25]
it is a “soft keyword”: it is not a reserved name, and can be used as a name for methods and local variables:
it = 5 # no warnings def it(&block) # RSpec-like API, no warnings # ... end
it can be used as a local variable even in blocks that use it as an implicit parameter (though this style is obviously confusing):
[1, 2, 3].each { # takes a value of implicit parameter "it" and uses it to # define a local variable with the same name it = it**2 p it }
In a block with explicit parameters defined it usage raises an exception:
[1, 2, 3].each { |x| p it } # syntax error found (SyntaxError) # [1, 2, 3].each { |x| p it } # ^~ 'it' is not allowed when an ordinary parameter is defined
But if a local name (variable or method) is available, it would be used:
it = 5 [1, 2, 3].each { |x| p it } # Prints 5, 5, 5
Blocks using it can be nested:
%w[test me].each { it.each_char { p it } } # Prints "t", "e", "s", "t", "m", "e"
Blocks using it are considered to have one parameter:
p = proc { it**2 } l = lambda { it**2 } p.parameters # => [[:opt]] p.arity # => 1 l.parameters # => [[:req]] l.arity # => 1
Numbered parameters
Numbered parameters are another way to name block parameters implicitly. Unlike it, numbered parameters allow to refer to several parameters in one block.
%w[test me please].each { puts _1.upcase } # prints TEST, ME, PLEASE {a: 100, b: 200}.map { "#{_1} = #{_2}" } # => "a = 100", "b = 200"
Parameter names from _1 to _9 are supported:
[10, 20, 30].zip([40, 50, 60], [70, 80, 90]).map { _1 + _2 + _3 } # => [120, 150, 180]
Though, it is advised to resort to them wisely, probably limiting yourself to _1 and _2, and to one-line blocks.
Numbered parameters can’t be used together with explicitly named ones:
[10, 20, 30].map { |x| _1**2 }
# SyntaxError (ordinary parameter is defined)
Numbered parameters can’t be mixed with it either:
[10, 20, 30].map { _1 + it } # SyntaxError: 'it' is not allowed when a numbered parameter is already used
To avoid conflicts, naming local variables or method arguments _1, _2 and so on, causes an error.
_1 = 'test' # ^~ _1 is reserved for numbered parameters (SyntaxError)
Using implicit numbered parameters affects block’s arity:
p = proc { _1 + _2 } l = lambda { _1 + _2 } p.parameters # => [[:opt, :_1], [:opt, :_2]] p.arity # => 2 l.parameters # => [[:req, :_1], [:req, :_2]] l.arity # => 2
Blocks with numbered parameters can’t be nested:
%w[test me].each { _1.each_char { p _1 } }
# numbered parameter is already used in outer block (SyntaxError)
# %w[test me].each { _1.each_char { p _1 } }
# ^~
Public Class Methods
Source
static VALUE
rb_proc_s_new(int argc, VALUE *argv, VALUE klass)
{
VALUE block = proc_new(klass, FALSE);
rb_obj_call_init_kw(block, argc, argv, RB_PASS_CALLED_KEYWORDS);
return block;
}
Creates a new Proc object, bound to the current context.
proc = Proc.new { "hello" } proc.call #=> "hello"
Raises ArgumentError if called without a block.
Proc.new #=> ArgumentError
Public Instance Methods
Source
static VALUE
proc_compose_to_left(VALUE self, VALUE g)
{
return rb_proc_compose_to_left(self, to_callable(g));
}
Returns a proc that is the composition of this proc and the given g. The returned proc takes a variable number of arguments, calls g with them then calls this proc with the result.
f = proc {|x| x * x } g = proc {|x| x + x } p (f << g).call(2) #=> 16
See Proc#>> for detailed explanations.
Source
static VALUE
proc_eq(VALUE self, VALUE other)
{
const rb_proc_t *self_proc, *other_proc;
const struct rb_block *self_block, *other_block;
if (rb_obj_class(self) != rb_obj_class(other)) {
return Qfalse;
}
GetProcPtr(self, self_proc);
GetProcPtr(other, other_proc);
if (self_proc->is_from_method != other_proc->is_from_method ||
self_proc->is_lambda != other_proc->is_lambda) {
return Qfalse;
}
self_block = &self_proc->block;
other_block = &other_proc->block;
if (vm_block_type(self_block) != vm_block_type(other_block)) {
return Qfalse;
}
switch (vm_block_type(self_block)) {
case block_type_iseq:
if (self_block->as.captured.ep != \
other_block->as.captured.ep ||
self_block->as.captured.code.iseq != \
other_block->as.captured.code.iseq) {
return Qfalse;
}
break;
case block_type_ifunc:
if (self_block->as.captured.code.ifunc != \
other_block->as.captured.code.ifunc) {
return Qfalse;
}
if (memcmp(
((cfunc_proc_t *)self_proc)->env,
((cfunc_proc_t *)other_proc)->env,
sizeof(((cfunc_proc_t *)self_proc)->env))) {
return Qfalse;
}
break;
case block_type_proc:
if (self_block->as.proc != other_block->as.proc) {
return Qfalse;
}
break;
case block_type_symbol:
if (self_block->as.symbol != other_block->as.symbol) {
return Qfalse;
}
break;
}
return Qtrue;
}
Two procs are the same if, and only if, they were created from the same code block.
def return_block(&block) block end def pass_block_twice(&block) [return_block(&block), return_block(&block)] end block1, block2 = pass_block_twice { puts 'test' } # Blocks might be instantiated into Proc's lazily, so they may, or may not, # be the same object. # But they are produced from the same code block, so they are equal block1 == block2 #=> true # Another Proc will never be equal, even if the code is the "same" block1 == proc { puts 'test' } #=> false
Source
static VALUE
proc_compose_to_right(VALUE self, VALUE g)
{
return rb_proc_compose_to_right(self, to_callable(g));
}
Returns a proc that is the composition of this proc and the given g. The returned proc takes a variable number of arguments, calls this proc with them then calls g with the result.
f = proc {|x| x * x } g = proc {|x| x + x } p (f >> g).call(2) #=> 8
g could be other Proc, or Method, or any other object responding to call method:
class Parser def self.call(text) # ...some complicated parsing logic... end end pipeline = File.method(:read) >> Parser >> proc { |data| puts "data size: #{data.count}" } pipeline.call('data.json')
Invokes the block, setting the block’s parameters to the arguments using something close to method calling semantics. Returns the value of the last expression evaluated in the block.
a_proc = Proc.new {|scalar, *values| values.map {|value| value*scalar } } a_proc.call(9, 1, 2, 3) #=> [9, 18, 27] a_proc[9, 1, 2, 3] #=> [9, 18, 27] a_proc.(9, 1, 2, 3) #=> [9, 18, 27] a_proc.yield(9, 1, 2, 3) #=> [9, 18, 27]
Note that prc.() invokes prc.call() with the parameters given. It’s syntactic sugar to hide “call”.
For procs created using lambda or ->() an error is generated if the wrong number of parameters are passed to the proc. For procs created using Proc.new or Kernel.proc, extra parameters are silently discarded and missing parameters are set to nil.
a_proc = proc {|a,b| [a,b] } a_proc.call(1) #=> [1, nil] a_proc = lambda {|a,b| [a,b] } a_proc.call(1) # ArgumentError: wrong number of arguments (given 1, expected 2)
See also Proc#lambda?.
Source
static VALUE
proc_arity(VALUE self)
{
int arity = rb_proc_arity(self);
return INT2FIX(arity);
}
Returns the number of mandatory arguments. If the block is declared to take no arguments, returns 0. If the block is known to take exactly n arguments, returns n. If the block has optional arguments, returns -n-1, where n is the number of mandatory arguments, with the exception for blocks that are not lambdas and have only a finite number of optional arguments; in this latter case, returns n. Keyword arguments will be considered as a single additional argument, that argument being mandatory if any keyword argument is mandatory. A proc with no argument declarations is the same as a block declaring || as its arguments.
proc {}.arity #=> 0 proc { || }.arity #=> 0 proc { |a| }.arity #=> 1 proc { |a, b| }.arity #=> 2 proc { |a, b, c| }.arity #=> 3 proc { |*a| }.arity #=> -1 proc { |a, *b| }.arity #=> -2 proc { |a, *b, c| }.arity #=> -3 proc { |x:, y:, z:0| }.arity #=> 1 proc { |*a, x:, y:0| }.arity #=> -2 proc { |a=0| }.arity #=> 0 lambda { |a=0| }.arity #=> -1 proc { |a=0, b| }.arity #=> 1 lambda { |a=0, b| }.arity #=> -2 proc { |a=0, b=0| }.arity #=> 0 lambda { |a=0, b=0| }.arity #=> -1 proc { |a, b=0| }.arity #=> 1 lambda { |a, b=0| }.arity #=> -2 proc { |(a, b), c=0| }.arity #=> 1 lambda { |(a, b), c=0| }.arity #=> -2 proc { |a, x:0, y:0| }.arity #=> 1 lambda { |a, x:0, y:0| }.arity #=> -2
Source
static VALUE
proc_binding(VALUE self)
{
VALUE bindval, binding_self = Qundef;
rb_binding_t *bind;
const rb_proc_t *proc;
const rb_iseq_t *iseq = NULL;
const struct rb_block *block;
const rb_env_t *env = NULL;
GetProcPtr(self, proc);
block = &proc->block;
if (proc->is_isolated) rb_raise(rb_eArgError, "Can't create Binding from isolated Proc");
again:
switch (vm_block_type(block)) {
case block_type_iseq:
iseq = block->as.captured.code.iseq;
binding_self = block->as.captured.self;
env = VM_ENV_ENVVAL_PTR(block->as.captured.ep);
break;
case block_type_proc:
GetProcPtr(block->as.proc, proc);
block = &proc->block;
goto again;
case block_type_ifunc:
{
const struct vm_ifunc *ifunc = block->as.captured.code.ifunc;
if (IS_METHOD_PROC_IFUNC(ifunc)) {
VALUE method = (VALUE)ifunc->data;
VALUE name = rb_fstring_lit("<empty_iseq>");
rb_iseq_t *empty;
binding_self = method_receiver(method);
iseq = rb_method_iseq(method);
env = VM_ENV_ENVVAL_PTR(block->as.captured.ep);
env = env_clone(env, method_cref(method));
/* set empty iseq */
empty = rb_iseq_new(Qnil, name, name, Qnil, 0, ISEQ_TYPE_TOP);
RB_OBJ_WRITE(env, &env->iseq, empty);
break;
}
}
/* FALLTHROUGH */
case block_type_symbol:
rb_raise(rb_eArgError, "Can't create Binding from C level Proc");
UNREACHABLE_RETURN(Qnil);
}
bindval = rb_binding_alloc(rb_cBinding);
GetBindingPtr(bindval, bind);
RB_OBJ_WRITE(bindval, &bind->block.as.captured.self, binding_self);
RB_OBJ_WRITE(bindval, &bind->block.as.captured.code.iseq, env->iseq);
rb_vm_block_ep_update(bindval, &bind->block, env->ep);
RB_OBJ_WRITTEN(bindval, Qundef, VM_ENV_ENVVAL(env->ep));
if (iseq) {
rb_iseq_check(iseq);
RB_OBJ_WRITE(bindval, &bind->pathobj, ISEQ_BODY(iseq)->location.pathobj);
bind->first_lineno = ISEQ_BODY(iseq)->location.first_lineno;
}
else {
RB_OBJ_WRITE(bindval, &bind->pathobj,
rb_iseq_pathobj_new(rb_fstring_lit("(binding)"), Qnil));
bind->first_lineno = 1;
}
return bindval;
}
Returns the binding associated with prc.
def fred(param) proc {} end b = fred(99) eval("param", b.binding) #=> 99
Source
0
static VALUE
proc_call(int argc, VALUE *argv, VALUE procval)
{
/* removed */
}
Invokes the block, setting the block’s parameters to the arguments using something close to method calling semantics. Returns the value of the last expression evaluated in the block.
a_proc = Proc.new {|scalar, *values| values.map {|value| value*scalar } } a_proc.call(9, 1, 2, 3) #=> [9, 18, 27] a_proc[9, 1, 2, 3] #=> [9, 18, 27] a_proc.(9, 1, 2, 3) #=> [9, 18, 27] a_proc.yield(9, 1, 2, 3) #=> [9, 18, 27]
Note that prc.() invokes prc.call() with the parameters given. It’s syntactic sugar to hide “call”.
For procs created using lambda or ->() an error is generated if the wrong number of parameters are passed to the proc. For procs created using Proc.new or Kernel.proc, extra parameters are silently discarded and missing parameters are set to nil.
a_proc = proc {|a,b| [a,b] } a_proc.call(1) #=> [1, nil] a_proc = lambda {|a,b| [a,b] } a_proc.call(1) # ArgumentError: wrong number of arguments (given 1, expected 2)
See also Proc#lambda?.
Source
static VALUE
proc_curry(int argc, const VALUE *argv, VALUE self)
{
int sarity, max_arity, min_arity = rb_proc_min_max_arity(self, &max_arity);
VALUE arity;
if (rb_check_arity(argc, 0, 1) == 0 || NIL_P(arity = argv[0])) {
arity = INT2FIX(min_arity);
}
else {
sarity = FIX2INT(arity);
if (rb_proc_lambda_p(self)) {
rb_check_arity(sarity, min_arity, max_arity);
}
}
return make_curry_proc(self, rb_ary_new(), arity);
}
Returns a curried proc. If the optional arity argument is given, it determines the number of arguments. A curried proc receives some arguments. If a sufficient number of arguments are supplied, it passes the supplied arguments to the original proc and returns the result. Otherwise, returns another curried proc that takes the rest of arguments.
The optional arity argument should be supplied when currying procs with variable arguments to determine how many arguments are needed before the proc is called.
b = proc {|x, y, z| (x||0) + (y||0) + (z||0) } p b.curry[1][2][3] #=> 6 p b.curry[1, 2][3, 4] #=> 6 p b.curry(5)[1][2][3][4][5] #=> 6 p b.curry(5)[1, 2][3, 4][5] #=> 6 p b.curry(1)[1] #=> 1 b = proc {|x, y, z, *w| (x||0) + (y||0) + (z||0) + w.inject(0, &:+) } p b.curry[1][2][3] #=> 6 p b.curry[1, 2][3, 4] #=> 10 p b.curry(5)[1][2][3][4][5] #=> 15 p b.curry(5)[1, 2][3, 4][5] #=> 15 p b.curry(1)[1] #=> 1 b = lambda {|x, y, z| (x||0) + (y||0) + (z||0) } p b.curry[1][2][3] #=> 6 p b.curry[1, 2][3, 4] #=> wrong number of arguments (given 4, expected 3) p b.curry(5) #=> wrong number of arguments (given 5, expected 3) p b.curry(1) #=> wrong number of arguments (given 1, expected 3) b = lambda {|x, y, z, *w| (x||0) + (y||0) + (z||0) + w.inject(0, &:+) } p b.curry[1][2][3] #=> 6 p b.curry[1, 2][3, 4] #=> 10 p b.curry(5)[1][2][3][4][5] #=> 15 p b.curry(5)[1, 2][3, 4][5] #=> 15 p b.curry(1) #=> wrong number of arguments (given 1, expected 3) b = proc { :foo } p b.curry[] #=> :foo
Two procs are the same if, and only if, they were created from the same code block.
def return_block(&block) block end def pass_block_twice(&block) [return_block(&block), return_block(&block)] end block1, block2 = pass_block_twice { puts 'test' } # Blocks might be instantiated into Proc's lazily, so they may, or may not, # be the same object. # But they are produced from the same code block, so they are equal block1 == block2 #=> true # Another Proc will never be equal, even if the code is the "same" block1 == proc { puts 'test' } #=> false
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static VALUE
proc_hash(VALUE self)
{
st_index_t hash;
hash = rb_hash_start(0);
hash = rb_hash_proc(hash, self);
hash = rb_hash_end(hash);
return ST2FIX(hash);
}
Returns a hash value corresponding to proc body.
See also Object#hash.
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VALUE
rb_proc_lambda_p(VALUE procval)
{
rb_proc_t *proc;
GetProcPtr(procval, proc);
return RBOOL(proc->is_lambda);
}
Returns true if a Proc object is lambda. false if non-lambda.
The lambda-ness affects argument handling and the behavior of return and break.
A Proc object generated by proc ignores extra arguments.
proc {|a,b| [a,b] }.call(1,2,3) #=> [1,2]
It provides nil for missing arguments.
proc {|a,b| [a,b] }.call(1) #=> [1,nil]
It expands a single array argument.
proc {|a,b| [a,b] }.call([1,2]) #=> [1,2]
A Proc object generated by lambda doesn’t have such tricks.
lambda {|a,b| [a,b] }.call(1,2,3) #=> ArgumentError lambda {|a,b| [a,b] }.call(1) #=> ArgumentError lambda {|a,b| [a,b] }.call([1,2]) #=> ArgumentError
Proc#lambda? is a predicate for the tricks. It returns true if no tricks apply.
lambda {}.lambda? #=> true proc {}.lambda? #=> false
Proc.new is the same as proc.
Proc.new {}.lambda? #=> false
lambda, proc and Proc.new preserve the tricks of a Proc object given by & argument.
lambda(&lambda {}).lambda? #=> true proc(&lambda {}).lambda? #=> true Proc.new(&lambda {}).lambda? #=> true lambda(&proc {}).lambda? #=> false proc(&proc {}).lambda? #=> false Proc.new(&proc {}).lambda? #=> false
A Proc object generated by & argument has the tricks
def n(&b) b.lambda? end n {} #=> false
The & argument preserves the tricks if a Proc object is given by & argument.
n(&lambda {}) #=> true n(&proc {}) #=> false n(&Proc.new {}) #=> false
A Proc object converted from a method has no tricks.
def m() end method(:m).to_proc.lambda? #=> true n(&method(:m)) #=> true n(&method(:m).to_proc) #=> true
define_method is treated the same as method definition. The defined method has no tricks.
class C define_method(:d) {} end C.new.d(1,2) #=> ArgumentError C.new.method(:d).to_proc.lambda? #=> true
define_method always defines a method without the tricks, even if a non-lambda Proc object is given. This is the only exception for which the tricks are not preserved.
class C define_method(:e, &proc {}) end C.new.e(1,2) #=> ArgumentError C.new.method(:e).to_proc.lambda? #=> true
This exception ensures that methods never have tricks and makes it easy to have wrappers to define methods that behave as usual.
class C def self.def2(name, &body) define_method(name, &body) end def2(:f) {} end C.new.f(1,2) #=> ArgumentError
The wrapper def2 defines a method which has no tricks.
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static VALUE
rb_proc_parameters(int argc, VALUE *argv, VALUE self)
{
static ID keyword_ids[1];
VALUE opt, lambda;
VALUE kwargs[1];
int is_proc ;
const rb_iseq_t *iseq;
iseq = rb_proc_get_iseq(self, &is_proc);
if (!keyword_ids[0]) {
CONST_ID(keyword_ids[0], "lambda");
}
rb_scan_args(argc, argv, "0:", &opt);
if (!NIL_P(opt)) {
rb_get_kwargs(opt, keyword_ids, 0, 1, kwargs);
lambda = kwargs[0];
if (!NIL_P(lambda)) {
is_proc = !RTEST(lambda);
}
}
if (!iseq) {
return rb_unnamed_parameters(rb_proc_arity(self));
}
return rb_iseq_parameters(iseq, is_proc);
}
Returns the parameter information of this proc. If the lambda keyword is provided and not nil, treats the proc as a lambda if true and as a non-lambda if false.
prc = proc{|x, y=42, *other|} prc.parameters #=> [[:opt, :x], [:opt, :y], [:rest, :other]] prc = lambda{|x, y=42, *other|} prc.parameters #=> [[:req, :x], [:opt, :y], [:rest, :other]] prc = proc{|x, y=42, *other|} prc.parameters(lambda: true) #=> [[:req, :x], [:opt, :y], [:rest, :other]] prc = lambda{|x, y=42, *other|} prc.parameters(lambda: false) #=> [[:opt, :x], [:opt, :y], [:rest, :other]]
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# File bundled-src/rbs-3.10.0/lib/rbs/test.rb, line 108 def ruby2_keywords self end
Marks the proc as passing keywords through a normal argument splat. This should only be called on procs that accept an argument splat (*args) but not explicit keywords or a keyword splat. It marks the proc such that if the proc is called with keyword arguments, the final hash argument is marked with a special flag such that if it is the final element of a normal argument splat to another method call, and that method call does not include explicit keywords or a keyword splat, the final element is interpreted as keywords. In other words, keywords will be passed through the proc to other methods.
This should only be used for procs that delegate keywords to another method, and only for backwards compatibility with Ruby versions before 2.7.
This method will probably be removed at some point, as it exists only for backwards compatibility. As it does not exist in Ruby versions before 2.7, check that the proc responds to this method before calling it. Also, be aware that if this method is removed, the behavior of the proc will change so that it does not pass through keywords.
module Mod foo = ->(meth, *args, &block) do send(:"do_#{meth}", *args, &block) end foo.ruby2_keywords if foo.respond_to?(:ruby2_keywords) end
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VALUE
rb_proc_location(VALUE self)
{
return iseq_location(rb_proc_get_iseq(self, 0));
}
Returns the location where the Proc was defined. The returned Array contains:
(1) the Ruby source filename (2) the line number where the definition starts (3) the column number where the definition starts (4) the line number where the definition ends (5) the column number where the definitions ends
This method will return nil if the Proc was not defined in Ruby (i.e. native).
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static VALUE
proc_to_proc(VALUE self)
{
return self;
}
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static VALUE
proc_to_s(VALUE self)
{
const rb_proc_t *proc;
GetProcPtr(self, proc);
return rb_block_to_s(self, &proc->block, proc->is_lambda ? " (lambda)" : NULL);
}
Returns the unique identifier for this proc, along with an indication of where the proc was defined.
Invokes the block, setting the block’s parameters to the arguments using something close to method calling semantics. Returns the value of the last expression evaluated in the block.
a_proc = Proc.new {|scalar, *values| values.map {|value| value*scalar } } a_proc.call(9, 1, 2, 3) #=> [9, 18, 27] a_proc[9, 1, 2, 3] #=> [9, 18, 27] a_proc.(9, 1, 2, 3) #=> [9, 18, 27] a_proc.yield(9, 1, 2, 3) #=> [9, 18, 27]
Note that prc.() invokes prc.call() with the parameters given. It’s syntactic sugar to hide “call”.
For procs created using lambda or ->() an error is generated if the wrong number of parameters are passed to the proc. For procs created using Proc.new or Kernel.proc, extra parameters are silently discarded and missing parameters are set to nil.
a_proc = proc {|a,b| [a,b] } a_proc.call(1) #=> [1, nil] a_proc = lambda {|a,b| [a,b] } a_proc.call(1) # ArgumentError: wrong number of arguments (given 1, expected 2)
See also Proc#lambda?.